1. Field of the Invention
The present invention relates to an active-matrix liquid-crystal display device and a substrate for use in the device and, more particularly, to a structure of storage capacitors in a substrate of an active-matrix liquid-crystal display device having a double-scanning-line feature.
2. Description of the Related Art
Since active-matrix liquid-crystal display devices typically employ a data line for each column of an arrangement of pixels, a number of data drivers is required if the number of pixels per row is large. Since the data driver is relatively costly, the use of a large number of data drivers makes the entire device expensive. Furthermore, in this type of conventional art, a small display area, namely, a small liquid-crystal display panel is difficult to manufacture. The liquid-crystal display panel having a small display area needs a miniaturized terminal section for the data lines. The liquid-crystal display panel in the conventional art having a large number of data lines places a rigorous requirement for a narrow pitch of the data line terminal section. For this reason, the manufacturing of the data line terminal section is difficult, leading to a low production yield.
FIG. 11 and FIG. 12 show an active-matrix liquid-crystal display device, which is also filed in the Japanese Patent Office by the assignee of the present invention. The active-matrix liquid-crystal display device disclosed employs a smaller number of data lines to drive pixels compared to the other conventional devices.
FIG. 11 and FIG. 12 show two examples of equivalent circuits of active-matrix liquid-crystal devices having half as many data lines as that in other conventional art devices. Each pixel is surrounded by a one-dot chain line. A data line Dj is shared by two pixels PX(i,j) and PX(i,j+1) (i=1, . . . , m for both pixels) in two columns with the data line Dj interposed therebetween. With this arrangement, the number of data lines is halved, and the number of data drivers is also halved.
As for each row, two adjacent pixels connected to the single data line Dj, for example, PX(i,j) and PX(i,j+1), need to be driven by separate gate lines GAi and GBi. For this reason, the number of gate lines becomes twice as many as that in the conventional art (this wiring method may be called a double-scanning-line method). The increase in the number of gate drivers does not increase the cost of the device, because the gate driver is substantially less costly than the data driver. Referring to FIG. 11 and FIG. 12, two adjacent pixels between two adjacent data lines are driven by different gate lines. Referring to FIG. 11, all dots on one side of one data line are connected to either GA gate lines or GB gate lines. Referring to FIG. 12, dots on one side of one data line are alternately connected to GA gate lines and GB gate lines.
Referring to FIG. 13, there is shown an actual structure of the above active-matrix liquid-crystal display device, in which two pixels are surrounded by two data lines and two gate lines. As shown, a thin-film transistor (hereinafter referred to as a TFT) 51 is formed on the top side of a right pixel D5, and on the bottom side of a left pixel D6. The two pixels D5 and D6 are arranged in a point symmetry fashion. With this arrangement, a gate line 52 widens in its portion at the TFT 51, serving as a gate electrode for the TFT 51. A semiconductor active layer 53 is formed on the gate electrode. Formed on the semiconductor active layer 53 are a source electrode 55, extending from a data line 54, and a drain electrode 56 with a spacing being maintained therebetween. The drain electrode 56 is electrically connected to a pixel electrode 58 via a contact hole 57.
In the active-matrix liquid-crystal display device, each pixel needs a storage capacitor to hold a signal supplied thereto for one scanning period. The gate line 52 is greatly increased in width on the side of each pixel opposite to the TFT 51, and a capacitor electrode 60, which is electrically connected to the pixel electrode 58 via a contact hole 59, is overlapped on a wide portion 52a of the gate line 52. A storage capacitor 61 is formed of the capacitor electrode 60, the wide portion 52a of the gate line 52, and an insulating layer interposed between the capacitor electrode 60 and the wide portion 52a. A rectangular shape 62 represented by a one-dot chain line in FIG. 13 shows an aperture of a black matrix formed on a counter substrate (not shown).
To achieve a desired storage capacitance in the active-matrix liquid-crystal display apparatus, the capacitor electrode 60 needs to be a sufficient area. For this reason, part of the gate line 52 is widened. The wide portion 52a of the gate line 52 for forming the storage capacitor 61 causes a narrow portion 52b in the gate line 52 at the adjacent pixel. The narrow portion 52b, shown in FIG. 13, does not overlapped with another electrode, and does not contribute to the storage capacitance. The wide portion 52a thus needs to compensate for the effect of the narrow portion. As seen from the area of the aperture 62 of the black matrix represented by the one-dot chain line in FIG. 13, the wide portion 52a of the gate line 52 reduces the aperture ratio of the panel.
In the gate line 52, the narrow portion 52b is much narrower than the wide portion 52a, increasing wiring resistance.